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CNPA: Wireless Networking

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Wireless Networking

This domain tests 802.11 generations, RF channel planning, association and wireless security. Items are usually practical: pick the non-overlapping channel, choose the correct security standard, or explain a hidden-node symptom.

Standards at a glance

  • 802.11b: 2.4 GHz, 11 Mbps. 802.11g: 2.4 GHz, 54 Mbps. 802.11a: 5 GHz, 54 Mbps.
  • 802.11n (Wi-Fi 4): 2.4 and 5 GHz, MIMO, up to 600 Mbps with 40 MHz channels.
  • 802.11ac (Wi-Fi 5): 5 GHz, wider channels, downlink MU-MIMO.
  • 802.11ax (Wi-Fi 6/6E): 2.4, 5 and 6 GHz, OFDMA, 1024-QAM, designed for high client density.

RF and channels

  • 2.4 GHz channels are spaced 5 MHz apart while a 20 MHz channel is far wider, so only 1, 6 and 11 are non-overlapping in the 1-11 regulatory set.
  • 5 GHz offers many more channels; some are DFS and must vacate when radar is detected.
  • Higher frequencies suffer more free-space loss: 5 GHz gives more channels and less interference but shorter range and poorer wall penetration.
  • 2.4 GHz interference sources include microwave ovens, Bluetooth, cordless phones and neighbouring APs.
  • 802.11 uses CSMA/CA with link-layer acknowledgements; RTS/CTS mitigates the hidden-node problem.

Joining and securing

  • Sequence: probe → authenticate → associate. BSSID is the AP radio MAC, SSID is the network name, and an ESS is multiple APs sharing an SSID for roaming.
  • WEP (RC4, broken) → WPA (TKIP) → WPA2 (AES-CCMP, 802.11i) → WPA3 (SAE, forward secrecy, protected management frames).
  • Personal/PSK suits small sites; Enterprise uses 802.1X with EAP against a RADIUS server for per-user credentials.

Worked example

A warehouse runs four APs on 2.4 GHz set to channels 1, 4, 8 and 11. Channel 4 overlaps both 1 and 8, and channel 8 overlaps 11, so neighbouring cells compete for airtime and clients show retries and low throughput. With only three clean channels available, the fix is to place APs so adjacent coverage cells use 1, 6 and 11 in a repeating pattern, lower transmit power to shrink cells rather than adding channels, and move high-demand clients to 5 GHz.

Traps

  • Choosing channel 3 because any number between 1 and 11 seems acceptable.
  • Mixing up TKIP (WPA) with AES-CCMP (WPA2).
  • Assuming 40 MHz channels help in 2.4 GHz; they consume most of the band.
  • Treating hidden node as an encryption problem rather than an RF and RTS/CTS problem.
  • Expecting 5 GHz to cover further than 2.4 GHz.